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Supraventricular Tachycardia (SVT)

Supraventricular tachycardia (SVT) is a narrow complex tachycardia, and it includes:

  • Sinus tachycardia

  • Atrial fibrillation

  • Atrial flutter

  • AVRT (AV Re-entry Tachycardia)

  • AVNRT (AV Nodal Re-entry Tachycardia)


Sinus tachycardia

This is usually secondary to exercise, pain, stress, anxiety, anaemia, hyperthyroidism, salbutamol, and caffeine. Managed by treating the underlying cause.

“Sinus Tachycardia” © Ewingdo (Licensed under CC-BY 4.0) https://creativecommons.org/licenses/by/4.0/


AVRT (AV Re-entry Tachycardia)

With this, an accessory pathway (AP) is formed between the atria and ventricles, leading to pre-excitation of the ventricles, therefore causing a characteristic delta wave. It can be classified into:

  • Orthodromic (95%) - Impulse travels down AVN and then back up through the accessory pathway (faster pathway = narrower QRS).

  • Antidromic (5%) - Impulse travels down accessory pathway and then back up through the AVN (slower pathway = broader QRS).

“Electrical Pathway of the Heart” © U Bhalraam (Licensed under CC-BY 4.0) https://creativecommons.org/licenses/by/4.0/


The most common type of this is Wolff-Parkinson-White (WPW), which consists of 2 types:

  • Type A - AP is on the left side of the heart. Creates a +ve delta wave in V1 and V2.

  • Type B - AP is on the right side of the heart. Creates a -ve delta wave in V1 and V2.


Management:

  • As always, if unstable, DC cardioversion them

  • Vagal manoeuvres first

  • If it fails, give AVN blocker e.g. Adenosine


N.B. Avoid AVN blockers if the patient also has AF as it will lead to VF (impulse is forced to go through the accessory pathway).


AVNRT (AV Nodal Re-entry Tachycardia)

With this, an accessory pathway is formed within the AV node, therefore creating both a fast and slow pathway. In normal sinus rhythm, the impulse travels down both at the same time, but the fast pathway quickly enters the slow pathway to cancel out that impulse, therefore preventing a re-entry circuit. But, if an ectopic atrial impulse reaches the AV node while the fast pathway is still in its refractory period, the impulse only goes down the slow pathway. This leads to a re-entry circuit.

The most common type of this is a Slow-Fast AVNRT.


Management:

  • As always, if unstable, DC cardiovert them

  • Vagal manouveres first

  • If it fails, give transient AVN blocker e.g. Adenosine


N.B. Avoid AVN blockers if the pt also has AF as it will lead to VF (impulse is forced to go through the accessory pathway).


Wolff-Parkinson-White (WPW)

ECG pattern seen here includes a Short PR interval and Delta wave. It has 2 types:

  • Type A - AP is on the left side of the heart. Creates a +ve delta wave in V1 and V2.

  • Type B - AP is on the right side of the heart. Creates a -ve delta wave in V1 and V2.

“Wolff-Parkinson-White Syndrome” © James Heilman (Licensed under CC-BY 3.0) https://creativecommons.org/licenses/by/3.0/


The 2 main forms of tachyarrhythmias that can occur here are:

  • Atrial fibrillation/flutter (20% of patients) - Due to direct conduction from atria to ventricles via AP (bypasses the AVN)

    • A HR > 200 bpm is way too fast to be conducted by the AVN

    • AF + WPW is usually mistaken for AF + LBBB. This can be distinguished by:

      • A variable beat-to-beat QRS width = WPW

      • A fixed beat-to-beat QRS width = LBBB

  • AVRT - Due to the formation of a re-entry circuit involving the AP and AVN. It can either be orthodromic or antidromic, based on the direction of conduction. Both usually have rates above 200 bpm.




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